As Temperature Rises, Earth Breathes Faster — and Maybe Harder

The critical question is whether soils release more CO2 because faster-growing plants pump more in, or if soils release CO2 that would have stayed in the ground at lower temperatures.

If the latter, the fresh influx of CO2 could produce a self-reinforcing cycle, producing higher temperatures that cause even more CO2 to be released.

“That’s the $50,000 question: Is there a feedback effect?” said Ben Bond-Lamberty, a University of Maryland, College Park biogeochemist and co-author of the review, in the March 24 Nature. “The data we have implies a feedback. It doesn’t prove it, but it’s consistent with the possibility.”

Carbon dioxide enters the soil through the roots of living plants and from the decaying bodies of dead plants, and is processed by microbes, fungi and insects. Over time, some of that CO2 releases back into the atmosphere. At any given time, there’s about twice as much CO2 in Earth’s soils as in its atmosphere.

Because more heat means more energy and faster chemical reactions, Earth scientists have suspected that rising global temperatures would increase the rate of soil respiration. The last review of soil respiration studies (.pdf) took place in 1992, however, and though it found a link between temperature and respiration rates, data was relatively sparse.

In the Nature paper, Bond-Lamberty and fellow UMCP geoscientist Allison Thomson combed the scientific literature for every controlled study of soil respiration published since 1960. They found 439 studies altogether, three-quarters of which were published after the 1992 review. When they analyzed the cumulative data, Bond-Lamberty and Thomson found that soil respiration increased by about 0.1 percent every year since 1989, and was tightly tied to temperature.

“The global soil-respiration flux is changing,” said Bond-Lamberty.

“The results of the analysis are consistent with previous estimations, but are more empirically based,” said James Raich, an Iowa State ecologist and co-author of the 1992 review.

The data did contain an anomaly. While respiration tracked with temperature in temperate and tropical regions, there was a negative correlation in the Arctic, where sensitivity to warming is thought to be especially pronounced.

According to Bond-Lamberty, researchers might have a flawed understanding of how carbon cycling works in Arctic soil. Those studies might also have been technically flawed. Because there were many fewer studies conducted in the Arctic than elsewhere, the results may be prone to statistical aberrations.

Whatever the anomaly’s explanation, that data was still included when global soil-respiration rates were calculated and the rise identified.

What’s not clear from the analysis is whether soil-respiration rates have increased without actually affecting atmospheric balances of CO2, or if CO2 that would have remained earthbound is now being released.

Both possibilities may be true, wrote University of Aberdeen biologist Pete Smith and Fudan University ecologist Changming Fang in a commentary accompanying the analysis.

“Assessing the balance between increased soil carbon inputs through greater plant growth due to climate warming, and increased carbon losses through higher decomposition rates, should be a research priority,” they wrote.

A small subset of studies in the review did try to answer that question by experimentally manipulating how much carbon entered test plots of soil. “That dataset is more tentative, but it does imply a feedback,” said Bond-Lamberty.